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The eerie effect was the product of a special atmospheric condition called a “superior mirage.” Superior mirages occur when a large area of warm air sits over a layer of colder air, a phenomenon called a temperature inversion. They’re actually common in the Arctic, where abnormal temperature gradients are more prevalent. As light rays pass through a temperature inversion, they are bent downward due to the higher density of the cold air, resulting in an inverted image that appears to float above its actual position.

Sometimes an object below the horizon may even become visible. In this case, the optical ray curvature was stronger than the curvature of Lake Michigan’s surface, which made it possible for Super and Blackmer to see the city’s buildings—even from across the Great Lake.

Super posted his snapshot of the Chicago superior mirage on Reddit, and it’s received nearly 1.5 millions views since.

Here’s Keith Matheny of the Detroit Free Press, interviewing Andrew T. Young, a San Diego State University astronomer and mirage expert:

Superior mirages are not uncommon at this time of the year, when the air is warming but lakes are still cold from the winter, Young said.

“The air isn’t usually clear enough to make the mirage visible at this big a range; so the clarity of the air is probably the most unusual circumstance in this case,” he said. “It’s also helpful that the setting sun is lighting up the high clouds in the sky beyond Chicago, so that the buildings are silhouetted against the bright sunset sky.”

A mirage isn’t an illusion, said Alistair Fraser, an emeritus meteorology professor from Pennsylvania State University who has studied the phenomenon.

“It’s an image that’s created as the atmosphere acts as a lens,” he said. “It’s kind of a crummy lens, so the images are like a carnival fun house full of twisted mirrors — to the point that the original object has been so greatly distorted, you can’t recognize what it is.”

People have been reporting superior mirages for centuries, though sightings are often of boats instead of entire cities. The mythical ghost ship the Flying Dutchman , for example, may be an example of a superior mirage.